The present invention relates to a nonaqueous electrolyte battery incorporating a case constituted by a laminate film and accommodating a battery device, and more particularly to an improvement in the portions in each of which an electrode terminal lead and the case are welded to each other with heat.
In recent years, a multiplicity of portable electronic apparatuses, such as camcoders, portable telephones and portable computers have made their appearance in recent years. The size and weight of the electronic apparatuses have been required to be reduced. As a portable power source for the above-mentioned electronic apparatuses, research and development of batteries, secondary batteries and nonaqueous electrolyte secondary batteries (so-called lithium-ion batteries) have energetically been performed to realize a thin and foldable structure.
Research and development of solidified electrolytic solution have energetically been performed to serve as the electrolyte of the above-mentioned shape-variable battery. In particular, a gel electrolyte which is a solid electrolyte containing a plasticizer and a polymer-type solid electrolyte having a structure that lithium salt is dissolved in polymers have received attention.
To use the merit of the foregoing battery that the thin and light weight structure can be realized, a variety of batteries have been studied which are structured such that a plastic film or a so-called laminate film obtained by bonding a plastic film and a metal member is used to seal the battery device. One of important requirements of the foregoing battery is to realize reliability of hermetic sealing as well as or better than that of the metal can.
For example, a battery disclosed in Japanese Patent Laid-Open No. 56-71278 incorporates a case constituted by a plastic film made of only resin. In the foregoing case, there arises a problem in that the moisture content which has penetrated the resin is introduced into the structure. Moreover, the electrolytic solution penetrates the resin and volatilizes. Therefore, the plastic film constituted by only the resin is not suitable for a battery incorporating a solid electrolyte which contains organic solvent.
To overcome the foregoing problem, it is effective to employ an aluminum laminate pack incorporating metal foil as the film for forming the case. In the foregoing case, short circuit must be prevented. For example, an elongated electrode terminal is taken from the interface of the case which is welded with heat. At this time, contact occurs between the end surface of the metal film of the case which is exposed to the outside over the opening for taking the electrode terminal and the electrode terminal. Also contact occurs between the surface of the metal film exposed to the outside owing to flow of the resin occurring when heat welding has been performed at an excessively high temperature or the pressure and the electrode terminal. Thus, short circuit occurs.
Therefore, for example, an attempt has been disclosed in Japanese Patent Laid-Open No. 9-288998 in which the electrode terminal is taken from the sheet-shape battery pack such that a lead wire coated with polyolefin denatured with maleic acid is employed. Moreover, also the resin for sealing the pack is the polyolefin denatured with maleic acid. Thus, an attempt to improve the adhesive properties is made.
The structure in which the portion of the lead which is welded to the case film with heat is coated with the sealant resin similar to the material of the film so as to improve the adhesivity. However, the apprehension that the sealant resin can be broken owing to heat and pressure cannot be eliminated. A method for preventing short circuit occurring owing to the breakage of the film may be employed with which the process is performed (the heat welding process) at a lower temperature. In the foregoing case, insufficiently low pressure causes a defect to take place with which prevention of burrs formed owing to cutting of the leads cannot be prevented and sealing of the leads along the two side ends cannot be performed. In case of the foregoing defect takes place, the airtightness of the sheet-shape battery pack greatly deteriorates.
As described above, a heat welding method which is capable of realizing both of satisfactory sealing characteristic and resistance against short circuit has not been realized as yet. Therefore, the foregoing method must be developed.